Since you say the soil seems compacted, I think I'd go for the aeration rather than the dethatching. It won't get rid of all the thatch, but it will poke holes in it and help the decomposition process. If you're mulch mowing, that should help get rid of the thatch, as well. There are some warm season grasses where mulch mowing can contribute to thatch, but since Marathon is a fescue, mulch mowing will help the thatch decompose. Over fertilizing and frequent shallow watering are the two biggest contributors to thatch, so if you're watering deeply and infrequently and fertilizing appropriately (especially with organic fertilizers), I think the thatch will start to go away on its own.

I'm going to disagree on the variety of grass. If you have thatch you don't have fescue. Thatch is not likely to develop in fescue under typical growing conditions. Thatch acumulates from dead stolons and rhizomes found in Bent grasses, Blue grasses and almost every warm season grass. Therefore de-thatching which is very agressive would not benefit your situation.

bpgreen mentioned that too frequent fertilizing can cause thatch. I've read here that with Bermuda (I do) you should fertilize about once per month in the summer. Is that going to cause thatch?

I can take this question since I grow bermuda for a living. Yes it will get thatchy However, for bermuda you need to fertilize that often. I would aerate a bermuda yard now so the holes will heal before the winter unless you overseed. Then do it in the fall if you must overseed.